The present invention is thus directed to the direct reduction of iron oxide particles to produce direct reduced iron (DRI), also known in the industry as sponge iron. More specifically, the present invention is directed to improvements in the methods and equipment employed for recycling cooling gas to the cooling zone of a direct reduction reactor.
For the nearly fifty years of commercial production DRI, the reducing and cooling gases have been recirculated by means of large and expensive compressors.
In the present invention, the normal compressor utilized for circulating the stream of cooling gas through the lower portion of a direct reduction reactor is preferably replaced, or at least supplemented, by at least one gas ejector; resulting unexpectedly in previously unrecognized and surprisingly effective important technical and economic advantages. As discussed in greater detail below, savings in the millions of dollars in reduced large initial capital costs and as well as greatly reduced operational savings in electrical energy and upkeep result.
Direct reduction processes are currently utilized in the steel industry as an alternative to blast furnaces, mainly in areas where natural gas is available and relatively low-capacity steelmaking plants are desired. Direct reduction processes of the moving bed type are exemplified by U.S. Pat. Nos. 4,049,440; 4,556,417; 5,858,057; 5,110,350; 5,437,708; and 6,319,301. In these processes, iron-containing particles in the form of pellets, lumps or mixtures thereof are charged to a reduction zone in the upper part of a moving bed shaft-type reduction reactor where the iron oxides of the ore particles are chemically reduced to metallic iron by means of a high-temperature reducing gas. The reduced and metallized iron particles or DRI (direct reduced iron) at a temperature above about 800° C. are discharged from said reduction reactor through a cooling zone in its lower part, where commonly they are cooled down to about ambient temperature to prevent re-oxidation of the DRI when discharged and contacted with air. To this end, a non-oxidizing gas stream is circulated in a cooling loop comprising the cooling zone of the reactor, a gas cooler for extracting the heat taken by the cooling gas from the hot DRI, a cooling gas compressor for recycling a major portion of said cooling gas back to said cooling zone and suitable connecting piping and control means for a continuous operation of said cooling gas loop. Usually, the gas utilized as cooling medium is natural gas mixed with reducing gas containing hydrogen and carbon monoxide. Even if pure natural gas is circulated as cooling gas through the cooling zone, the hydrocarbons present in the natural gas (methane, ethane, propane, aromatics, etc) will in some proportion, depending on the temperatures and time encountered, partially transform into hydrogen and carbon monoxide through the catalytic action of the DRI at the temperatures encountered by said cooling gas in the cooling zone.
Gas ejectors are currently utilized in several industrial processes and plants for moving gas streams. Some examples of suggested ejector applications are described below.
German Laid-Open Patent Applications Nos. DE 4010602 and DE 4010603 describe a process for ammonia production where an ejector is used for upstream recycling of a gas stream using a higher pressure gas stream from the same process. The processes shown in these applications are different and in both of them an ejector is utilized instead of a compressor for increasing the pressure of a gas so that it can be fed to the first steps of the process.
U.S. Pat. No. 6,508,998 issued to Nasato discloses the use of an ejector in a Claus plant for sulfur recovery from a hydrogen sulfide containing gas. An ejector is employed for recycling a portion of the gas effluent from the oxidation and heat-recovery vessels to the burner of the oxidizing reactor. The motive fluid for the ejector may be selected from steam, air, nitrogen, carbon dioxide, sulfur dioxide or other compatible gases.
U.S. Pat. No. 6,818,198 issued to Singh et al. discloses an autothermal reforming process wherein a portion of the syngas produced is recycled to the reformer utilizing an ejector 124 wherein the motive fluid stream is a mixture of hydrocarbon and steam. In this case the ejector also serves as a mixer of the feed gas to the reformer and recycled syngas product. Syngas recycling increases the proportion of hydrogen in the product gas.
U.S. Pat. No. 4,325,731, issued Apr. 20, 1982 (with a 1979 priority date) is the only one known to Applicant and his assignee that even has a suggestion of any use of an ejector in conjunction with a DRI reduction reactor. This patent is completely silent as regards any cooling of the DRI.
This patent only incidentally shows an ejector 7 using gasifier product 6 as the motive gas to mix with and feed a reformer 10 with scrubbed depleted reducing gas 8 from a reduction reactor 12. This feature was not claimed. Even the suggested use of ejectors apparently has not proven practical (for recycling reducing gas). Contrary to the suggestion in this quarter-century old reference, Applicant and his assignee are not aware that this suggestion has ever been commercially implemented or proven to have been effective or beneficial in such an application.
More to the point, there is no suggestion whatsoever in this patent to use an ejector in the cooling gas recycle loop of a DRI reduction plant, much less any recognition of the many advantages thereof; especially including being able to take advantage of the high pressure natural gas supply that is furnished from outside merchant sources and thus exist as a “free” motive energy source. All this is in spite of such pressurized natural gas having been used as a feed stock source in commercial DRI plants for almost 50 years and never before having been taken advantage of in this way.
It should be noted that a given volume of natural gas, upon being reformed, gives a gas that increases in volume four times. Thus, if the volume of gas fed as make-up feed stock to the reducing gas loop in a commercially viable DRI plant is insufficient to drive an ejector to circulate the recycle reducing gas; then this insufficiency would be even greater where the make-up is a fractionally smaller volume of unreformed natural gas. Yet the Applicant surprisingly has found that the volume of make up, even in the form of natural gas, when fed to the cooling zone is effective to drive ejector(s) sufficient to replace the compressor used in the cooling gas recycle loop. Another reason, that in hindsight can be seen to make this viable in the cooling zone, is that the pressure needed to drive the recycle gas in the cooling gas loop is typically on the order of less than one fifth of that needed to drive recycle gas in the reducing gas loop. The cooling gas loop essentially has only the cooling zone and the cooler/scrubber, while the reducing gas loop will at least in addition also have a heater and/or a reformer (see 10 in the '731 patent).
Documents cited in this text (including the foregoing patents), and all documents cited or referenced in the documents cited in this text, are incorporated herein by reference. Documents incorporated by reference into this text or any teachings therein may be used in the practice of this invention.
The present invention thus overcomes several shortcomings of the relevant prior art for the production of DRI, by utilizing for the first time an ejector in the cooling loop and where, for the first time in the almost 50-year commercial production of DRI, the high-pressure of the commonly-present natural gas make-up feed is utilized as the motive power for an ejector to mix with and drive any recycle gas. This invention in using ejector(s) replaces the conventional large mechanical compressor for circulating cooling gas; or, in existing installations, at the very least gives relatively inexpensive support for such a compressor and thus additionally avoids the capital and upkeep costs of a no longer needed, dramatically more expensive, stand-by or supplemental compressor).